Duct materials | Galvanized steel, fiberglass insulated duct, corrugated aluminum, or flexible spiral-wound Mylar [15]; iron, steel, aluminum, concrete, masonry, clay tile, fibrous glass, or G90-coated galvanized ducts | Steel, aluminum, or a material with a UL Standard 181 listing. | Galvanized steel, uncoated carbon steel, or aluminum | Carbon steel, type 304/430 stainless steel | Thermoplastic and fiberglass reinforced thermosetting ducts |